Fix: Graph deleted before aptly exits

[reglim]

Fixes: #1213

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

The temporary output file is now only deleted after copying it to the output location.
We no longer wait 1s for the open command, but exit immediately, since the process will continue running even after aptly exits.

Why this change is important?

Because the assumption was made that any application that opened the image would continue displaying it, even if the image was removed right after, which isn't true for the Gnome Image Viewer.

Checklist

• unit-test added (if change is algorithm)
• functional test added/updated (if change is functional)
• man page updated (if applicable)
• bash completion updated (if applicable)
• documentation updated
• author name in AUTHORS

[codecov]

Codecov Report

Attention: 9 lines in your changes are missing coverage. Please review.

Comparison is base (f1649a6) 74.85% compared to head (7192f1e) 66.64%.
Report is 4 commits behind head on master.

❗ Current head 7192f1e differs from pull request most recent head 2ee09ab. Consider uploading reports for the commit 2ee09ab to get more accurate results

Files	Patch %	Lines
cmd/graph.go	18.18%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1214      +/-   ##
==========================================
- Coverage   74.85%   66.64%   -8.22%     
==========================================
  Files         143      143              
  Lines       16187    16184       -3     
==========================================
- Hits        12117    10786    -1331     
- Misses       3134     4630    +1496     
+ Partials      936      768     -168     

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